At present, a thin film of a high-melting point metal such as molybdenum (hereafter, Mo) which has low electrical resistance is being used in a thin film electrode and thin film wiring etc. of a flat-panel display such as a Liquid Crystal Display, and a target material for sputtering is widely used as a material for forming the metal thin film.
In producing a Mo bulk body such as a target material, a method of sintering Mo raw material powder to form a bulk body is generally adopted because Mo is a high-melting point metal and therefore it is difficult to produce the Mo bulk body by a melting and casting method. As one such method, a method in which Mo raw material powder obtained by reduction-treating Mo oxide is compression-molded, is sintered in a reducing atmosphere, and is further subjected to plastic working to increase the density thereof is adopted. On the other hand, a method in which Mo raw material powder obtained by reduction treatment is adjusted to have a constant particle size, is packed into a pressure vessel, and thereafter is subjected to a pressure sintering process to obtain a high-density sintered body.
Moreover, since Mo powder with high-purity, typically with a purity of not less than 99.9% (3N) is produced by reduction treatment of Mo oxide (MoO3) powder, the Mo powder is usually fine powder having a particle size of not more than 15 μm in which minute primary particles are aggregated. Therefore, a specific surface area per unit volume is large, a sufficient packing density is not achieved even if being packed in a vessel etc., and also, a non-uniform packing state is caused in the vessel, so that there has been a problem that deformation such as warpage of the sintered body is likely to occur during the pressure sintering.
In contrast to this, the present applicant has proposed a technique in which fine Mo raw material powder is once compression-molded and thereafter crushed to produce a secondary powder having a large particle diameter, and then the secondary powder is packed into a pressure vessel so as to be pressure-sintered, as a method for producing a Mo target in which deformation during sintering is prevented.
This method utilizes an advantage that the secondary powder having a large particle diameter is excellent in the packing property (JP-A-2008-163467, Patent Literature 1).
Further, there is also disclosed a method in which the Mo raw material powder is made to be coarse particles by reduction-treating Mo oxide to which a specific amount of potassium is added. This method uses Mo raw material powder which is made to be particles coarser than those in typical Mo powder obtained by reduction-treating Mo oxide, and thereby proposes to increase the sintered density when the Mo raw material powder is sintered (JP-A-2005-133197, Patent Literature 2).
Further, the present applicant has proposed a method in which a bulk body of a high-melting point metal such as Mo is cut out to obtain machining chips (turning chips), and the machining chips are subjected to crushing processing and reduction treatment to form low-oxygen Mo powder, as a method for producing Mo powder (JP-A-2006-193826, Patent Literature 3).